85th percentile speed

85th Percentile Speed

The concept of the 85th percentile speed is a fundamental principle in traffic engineering and road safety management. It is used to set speed limits and assess the safety of roadways. The 85th percentile speed is the speed at or below which 85% of vehicles are observed to travel under free-flowing conditions. This measure is based on the assumption that the majority of drivers are reasonable and prudent, and that they select speeds that are safe for the current road conditions.

Definition and Calculation

The 85th percentile speed is determined through a speed study, which involves measuring the speeds of a sample of vehicles on a particular road segment. The data collected is then analyzed to find the speed below which 85% of the vehicles are traveling. This speed is considered to reflect the behavior of the majority of drivers and is used as a benchmark for setting speed limits.

Importance in Traffic Engineering

The 85th percentile speed is a critical factor in traffic engineering for several reasons:

• Setting Speed Limits: It is commonly used to establish speed limits that are both safe and reasonable. Speed limits set at or near the 85th percentile speed are believed to minimize the variance in vehicle speeds, which can reduce the likelihood of accidents.

• Road Safety: By aligning speed limits with the natural driving behavior of the majority, the 85th percentile speed helps in reducing speed differentials among vehicles, which is a key factor in preventing collisions.

• Traffic Flow: Roads designed with the 85th percentile speed in mind tend to have smoother traffic flow, as most drivers are traveling at similar speeds.

Criticism and Limitations

While the 85th percentile speed is widely used, it is not without criticism:

• Changing Conditions: The 85th percentile speed does not account for changes in road conditions, such as weather, visibility, or traffic congestion.

• Pedestrian and Cyclist Safety: Critics argue that setting speed limits based solely on vehicle speeds can neglect the safety of pedestrians and cyclists.

• Urban Areas: In densely populated urban areas, the 85th percentile speed may not be appropriate due to the higher presence of non-motorized road users and frequent intersections.

Applications

The 85th percentile speed is applied in various aspects of traffic management, including:

• Speed Limit Reviews: Regular reviews of speed limits on existing roads often use the 85th percentile speed as a key metric.

• Road Design: New road designs may incorporate the 85th percentile speed to ensure that the road can safely accommodate the expected traffic speeds.

• Traffic Calming Measures: In areas where the 85th percentile speed is deemed too high for safety, traffic calming measures such as speed bumps or chicanes may be implemented.

Also see

• Speed Limit
• Traffic Engineering
• Road Safety
• Traffic Calming
• Pedestrian Safety